The method of this invention has general applicability to any aqueous system where the formation and deposition of scale is a potential problem. It is particularly useful in inhibiting the formation and deposition of calcium orthophosphate scale. The most common systems in which scale formation can be a problem are in cooling water and boiling water systems. For the sake of simplicity, this invention will be discussed as it concerns cooling water systems.
A cooling water system is any system in which water is passed through a heat exchanger to provide cooling. A common system is a circulating system in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat through evaporation of a portion of the cooling water. One result of this type of system is that the dissolved solids and suspended solids in the water become concentrated. The term employed in the art to indicate the degree of concentration of the circulating water as compared with the make-up water that is added to the system to replace water that has evaporated is "cycles of concentration". For example, two cycles of concentration indicates that the circulating water contains twice the concentration of dissolved and suspended solids as the make-up water.
Deposits in lines, heat exchange equipment, etc. may originate from several causes. Contaminant particles or substances may enter the system, where they may act as foulants which may agglomerate on surfaces. Corrosion of the system may occur resulting in deposition of the corrosion products on surfaces within the system. Additionally, deposits may form as a result of crystallization or precipitation of salts from the solution. These crystallized or precipitated salts are referred to as "scale" and the process is referred to as "scale formation".
Many factors affect the rate and quantity of scale formation. Some of these factors are as follows, temperature, rate of heat transfer, pH of the water, and the character and amount of dissolved solids in the water.
Although the discussion above is centered primarily on cooling water systems, the same considerations also apply to boiler water systems, and many other aqueous systems. Any aqueous system having calcium or magnesium cations or other positive hardness ions, and phosphate, carbonate, sulfate, or other suitable anions can experience formation and deposition of scale.
Circulating water systems have been operated under acidic conditions because of increased solubility of scale forming salts under acidic pH. Corrosion inhibition under these conditions in most instances involved solutions of chromate compounds. Chromate compounds have come under tight control as a result of limitations on pollutant discharge and other environmental concerns. As a result, the operating conditions for most cooling water systems had to be modified to accommodate new corrosion inhibitors. Cooling water systems are now most commonly run at relatively high pH. Many current corrosion inhibition systems involve the use of phosphates or phosphonates. The change to higher pH in cooling water systems has greatly increased the possibility of precipitation of scale forming salts. Additionally, the use of phosphates as corrosion inhibitors has greatly increased the possibility of formation of calcium phosphate scale, particularly calcium orthophosphate scale. Calcium orthophosphate is highly insoluble and inhibition of calcium orthophosphate scale has proven to be very difficult.
In addition to encrustation and heat transfer problems associated with scale formation, formation of calcium phosphate scale also removes phosphate from solution, reducing its effectiveness as a corrosion inhibitor. Inhibition of formation of calcium orthophosphate scale allows a higher concentration of phosphate in solution resulting in better corrosion inhibition. In this way, a calcium phosphate scale inhibitor can also be thought of as enhancing corrosion inhibition.
Even if phosphates are not added for corrosion inhibition, formation of calcium phosphate scale can be a problem. Phosphates can also be present in the water from a number of other sources. The supply water may contain phosphates from fertilizer runoff, sewage treatment, photochemical or oxidative degradation of phosphonates, natural sources, and other sources.
Calcium carbonate and calcium sulfate scale have also become larger problems as the pH of cooling water has been increased. Calcium carbonate and calcium sulfate are both soluble in an acidic solution, but are relatively insoluble at the higher pH's currently used in circulating water systems.
Of the polymers useful for scale inhibition, only a very small proportion are effective for inhibition of calcium orthophosphate scale. In view of the above, inhibition of scale formation, particularly inhibition of calcium phosphate scale, is becoming a larger, more difficult problem.
U.S. Pat. No. 4,029,577, to Godlewski et al. discloses a class of polymers that are said to be effective at inhibiting formation of calcium orthophosphate scale. These polymers are copolymers of either acrylic acid or derivatives of acrylic acid with hydroxy lower alkyl acrylates. U.S. Pat. No. 4,326,980, which issued Apr. 27, 1982 to Snyder et al., discloses a composition which is said to be effective for inhibiting calcium orthophosphate scale. This composition is a mixture of the copolymer of the Godlewski patent and a water soluble alkyl phenoxy polyethoxyethanol.
U.S. Pat. No. 4,432,884 to Kawasaki et al. discloses a class of polymers that are said to be effective at inhibiting formation of calcium phosphate scale. These polymers are copolymers of specified alkylolamides with specified alkenyl compounds. Both the alkylolamides and the alkenyl compounds are actually derivatives of acrylic acid.
U.S. Pat. No. 4,435,556 to Masler discloses a class of terpolymers that are said to be effective at inhibiting formation of calcium phosphate scale. The terpolymer consists essentially of the two monomers of Godlewski along with a lower alkyl polyalkoxylated acrylate or methacrylate.
Each of the polymers of these references is, broadly speaking, an acrylate polymer. The identification of scale inhibitors that are effective at high pH's that are currently used, and particularly those that are effective against calcium orthophosphate scale, would be an advancement in the art.